Field of the Invention
The present invention relates to solid state laser modules and laser systems that incorporate solid state laser modules.
Description of Related Art
Laser devices that incorporate solid state lasers are used for a variety of applications including but not limited to applications such as target marking, pointing, designating, aiming, data communication, remote sensing and stand-off spectral analysis. These applications require that a laser beam be generated that travels through free space to create a laser spot on a distant target that reflects enough light to be observed or sensed by an electronic sensor such as a spectrometer, a spectrophotometer or an array type image sensor that may itself be a great distance from the target. Such applications require a laser system that can emit a laser beam with limited divergence.
In many of these applications, it is also necessary that such laser devices are in a form that is easily transported to a point of use and that is readily and reliably operated when needed. This requires laser devices that are lighter, have a smaller cross-sectional area and reduced volume while still being robust enough to operate after being exposed to vibration, tension, compression, bending forces, torsional loads, and environmental extremes during transport and use. It is particularly important in military, homeland security, and first responder applications that such laser devices will remain operational even when exposed to high levels of shock.
Bonding wires, also known as wire bonds, are frequently used for making electrical connections between fixed components of a laser module. For example, a common use of a wire bond is to join electrical terminals of the laser module the laser or to sensors within the module. Wire bonds are typically made using popular conductors such as gold, aluminum, and copper. These wire bonds are often ultrasonically, thermosonically, or thermally welded at each end to form a physical and electrical connection to the components of the laser module and extend unsupported through free space within the laser module.
It will be appreciated that when devices having such wire bonds are used in mobile products such handheld or otherwise portable products such laser modules may be exposed to ongoing vibrational accelerations of various types and intensities. As is noted, for example, in U.S. Pat. No. 6,668,667, such vibrations can create stress in the wire bonds and can cause deformation of the wire bonds including plastic deformation of the wire bonds. Over time, such vibrations can cause fatigue failures such as a failure known as a “heel crack” which occurs generally at a portion of the wire bond frequently at a kink point at or near a bonding point. The vibrational accelerations that such devices are exposed to are usually within a limited range of peak accelerations and testing techniques and devices such as those that are described in the '667 patent can be used to apply vibrational accelerations to such devices to simulate long the effects of such vibrational accelerations.
However, some applications require devices that can survive peak accelerations that are significantly larger than those associated with conventional vibrations. For example, and without limitation, weapons mounted laser systems, vehicle mounted laser systems, aircraft and other aerospace vehicle mounted laser systems can be exposed to higher order transient accelerations that are an order of magnitude or more greater than those of conventional vibrational accelerations during weapons discharge, during take-off, upon landing or in extreme maneuvering.
Such high levels of transient acceleration can induce separation of freely moving portions of the wire bond from bound portions of the wire bond and or can cause plastic deformation of the wire bond leading to increased resistance within the laser module. Additionally, such high levels of shock can cause wire bonds two separate from surfaces to which they are bound and can induce failure of materials in the surfaces to which the wire bond is mounted such that the wire bond remains attached to the substrate but the substrate itself fractures and separates from remaining portions of the substrate.
Such higher order transient accelerations can also bring separate wire bonds into temporary or permanent contact thereby disrupting electrical systems. The latter risk is particularly acute in laser modules to which multiple wire bonds are connected in relatively close proximity in order to provide desirable patterns of conductivity.
FIGS. 1 and 2 are a generally side elevation view and a top view of a person 10 with a hand held weapon system 12 having a laser device 14. As is shown in FIGS. 1 and 2, laser device 14 sits below hand held weapon system 12 and has a bore axis 20 along which a projectile fired by hand held weapon system 12 will travel. In the embodiment of FIGS. 1 and 2, bore axis 20 is aligned relative to a z-axis. Similarly, an optical axis 22 of laser 14 is aligned relative to the z-axis. In the embodiment that is illustrated, bore axis 20 and optical axis 22 are parallel to each other and to the z-axis. In other embodiments, there may be divergence between bore axis 20 and optical axis 22 so as to provide, for example, elevation adjustments and parallax adjustments.
When hand-held weapon system 12 is fired, the detonation of a charge in hand-held weapon system 12 accelerates a projectile (not shown) in a first direction 40 along the z-axis while hand-held weapon system 12 and laser system 14 experience acceleration in a second direction 42 along the z-axis. However, these are not the only accelerations experienced by hand-held weapon system 12 and laser device 14.
FIG. 3 is a side elevation view of person 10 and hand-held weapon system 12 immediately after discharge of a projectile (not shown) and illustrates that such discharge causes z-axis acceleration in second direction 42 and also causes y-axis acceleration 50 that can thrust hand-held weapon system 12 upwardly. In the case of hand-held weapon system 12, y-axis acceleration 50 causes hand-held weapon system 12 to move along an arcurate path 52. Alternatively, in some other circumstances, (not shown) y-axis acceleration 50 can cause handheld weapon system 12 to move downwardly. Here too, such downward movement may follow an arcurate path.
Additionally, as is shown in FIG. 4, discharge of a projectile (not shown) from hand-held weapon system 12 can also induce y-axis accelerations 60 or 62 which, in the case of hand-held weapon system 12 can cause side to side arcurate motions 66 and 64 of hand-held weapon system 12.
Z-axis, y-axis and x-axis accelerations at hand-held weapon system 12 can occur simultaneously, can overlap or can otherwise occur in ways that combine to provide transitory accelerations that disrupt operation of a laser module. Often such accelerations follow a sinusoidal acceleration and damping pattern such that hand-held weapon system 12 experiences positive and negative accelerations along the z-axis, y-axis and x-axis in the few milliseconds after discharge of a projectile.
The accelerations experienced by hand-held weapon system 12 are transmitted to laser module 14 and cause wire bonds used in the laser device to move in ways that can interrupt operations of laser device 14.
FIGS. 5 and 6 illustrate potential implications of the movement of wire bonds used in laser device 14 by illustrating one example of a prior art laser module 70. As is shown in FIG. 5, prior art laser device 70 has pins 72 that extend through a base 74 and a header 88 extending from base 74 to support a laser 76. A housing 78 encloses pins 72, header 88 and laser 76. An opening 82 in housing 78 has a lens 84 that focuses light from laser 76.
As is shown in FIG. 5, a first wire bond 90 extends from one of pins 72 to laser 76, a second wire bond 92 similarly extends from one of pins 72 to a sensor 86 and a third wire bond 94 extends from one of conductors 72 to header 88. It will be appreciated from FIG. 5 that first wire bond 90, second wire bond 92, and third wire bond 94 are joined only at respective ends of each wire bond and otherwise extend across significant distances through free space. Accordingly, exposure of laser device 70 to transient higher order accelerations can cause any of a range of failures at wire bonds 90, 92 and 94.
FIG. 6 illustrates a potential outcome when laser module 70 is exposed to transient higher order accelerations. As is shown in FIG. 6, these transient high order accelerations alter the shape and relative position of first wire bond 90 and second wire bond 92 such that first wire bond 90 and second wire bond 92 come into contact creating a short 96. As is also illustrated in FIG. 6, the shock to which prior art laser device 70 is exposed also causes a fracture 98 in third wire bond 94. Other failure modes are possible.
For these reasons, among others, semiconductor devices are subjected mechanical shock testing to demonstrate the survivability of the devices. One example of such a shock test is MIL-STD-833, Method 2002. This method requires among other things that semiconductor device be capable of surviving transient accelerations of up to 30,000 times the acceleration of gravity. However, an approach of simply testing and iteratively redesigning semiconductor devices with shorter or repositioned wire bonds that are calculated to survive such tests is expensive and time-consuming.
In one alternative approach, U.S. patent No. 2012/0027040 describes a system in which wire bonds are encased in a resin after manufacture in order to hold such wire bonds in a predetermined position relative to each other and relative to the laser module. However, such an approach traps heat within the laser module and cannot be effectively used with lasers that generate substantial amounts of heat.
In another alternative approach, U.S. Pat. No. 6,717,100 describes the use of ribbon type conductors to join components within an electrical system and suggests that in some cases the same ribbon can be bonded twice to structures to which it is joined so as to achieve better current distribution and a more reliable bond. This however requires joining the wire bonds to surfaces multiple times in quick succession which can itself cause damage to the surfaces or to the ribbon conductors. Additionally, this approach provides improved fixation of the wire bonds without addressing the risks associated with accelerating the wire bonds.
What is needed in the art therefore is a new approach to the manufacture of laser modules that allows the use of wire bond technology while limiting the effects that higher order acceleration can have on such wire bonds.